1. Field of the Invention
This invention relates to a process for recovering a subliming substance and more specifically to a process for recovering a subliming substance produced by a gas phase catalytic oxidation.
2. Description of Related Art
Pyromellitic anhydride is a subliming substance and is useful as the raw material mainly for a heat-resistant macromolecular substance such as polyimide resin or a curing agent for epoxy resins.
A technique for producing pyromellitic anhydride with high purity is disclosed in JP-A-10-265,474, which comprises introducing a pyromellitic anhydride-containing gas into a vertical recovery vessel provided with a surface for the deposition of crystals of pyromellitic anhydride, allowing the pyromellitic anhydride to be deposited in the form of crystals on the surface, and heating the surface to a temperature of 210xc2x0-260xc2x0 C. and thereby causing the crystals to peel from the surface and fall down.
JP-A-10-279,522 also discloses a recovery process which comprises causing pyromellitic anhydride to be deposited in the form of crystals on the surface for crystal deposition and then lowering the temperature of the surface below the temperature during the deposition of crystals thereon thereby causing the deposited crystals to peel off the surface and fall down.
When a gaseous substance is collected in a solid state, the efficiency of this collection will be enhanced as the temperature at which the collection proceeds is lowered. The lowering of the temperature of this collection, however, has heretofore entailed problems such as causing formed crystals to color and tend easily adhering to the surface used for the collection and, with the decrease in the temperature, suffering the crystals increasingly to gain in density, adherence to the surface, and difficulty of separation from the surface. This collection has been, therefore, performed at such a high temperature as enables the formed product to avoid coloration and allow relatively easy recovery in spite of a low efficiency of collection.
We have made a diligent study of the above problems and have consequently discovered that even crystals collected at a lower temperature than ever can be recovered fully by combining the steps of lowering and heightening the temperature of the surface used for collecting devices. This invention has been achieved as a result.
This invention is accomplished by a process for recovering a subliming substance in the form of crystals from a subliming substance-containing, which process is characterized by combining an operation of cooling the surface of the collecting tubes for deposition of crystals to a temperature lower than the temperature at which the crystals deposit with an operation of heating the surface to a temperature higher than the temperature at which the crystals deposit.
According to the process of this invention, the subliming substance can be recovered continuously with a high efficiency without coloring problem thereof by combining the cooling and heating operations even when a given subliming substance is collected at a lower temperature than ever.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.